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Apparatus for decoding an HDTV signal conveyed as high and low priority
data in high and low priority channels respectively, wherein said high and
low priority data originated from blocks of hierarchically encoded
compressed video data, with data of greater importance for image
reproduction from each block allocated to the high priority channel and
the remaining data from each block allocated to the low priority channel,
includes circuitry for identifying and segmenting high and low priority
data for corresponding blocks and recombining the high and low priority
data into an hierarchically encoded compressed video data stream.

1. In a television processing system for decoding compressed video signal encoded in such fashion that data of greater and lesser importance for image reproduction are
transmitted as high and low priority data in high and low priority channels respectively, said data including variable length codewords CW of differing types, said types occurring in a predetermined sequence of blocks of data, with data from respective
blocks occurring in both said high and low priority channels, and wherein the data of said high and low priority channels are conveyed in respective transport packets having header data including block identifiers, and the header data of the high
priority channel transport packets including codewords CW#j identifying numbers of codewords included in groups of one of said types, apparatus for combining said data from said high and low priority channels into a single data stream comprising:

first means, responsive to received signal, for providing said high and low priority data with header data excised, and for providing said header data;

second means responsive to said high priority data and said header data from said high priority channel for variable length decoding said high priority codewords CW to establish demarcation between respective codewords, and to identify respective
types of said codewords;

third means responsive to said low priority data and said header data from said low priority channel for variable length decoding said low priority codewords CW to establish demarcation between respective codewords, and to identify respective
types of said codewords;

fourth means, responsive to said header data, for identifying corresponding blocks of high and low priority data, and providing as said single data stream, high priority data codewords for a block through codeword number CW#j, then low priority
data for the same block.

2. The apparatus set forth in claim 1 wherein said second means includes means for associating codeword numbers CW#i with respective codewords according to codeword types, and said fourth means includes means for concurrently providing said high
priority codewords and associated codeword numbers CW#i and detecting the occurrence of the codeword number CW#i equal to CW#j.

3. The apparatus set forth in claim 1 wherein each block of low priority data includes an end of block code EOB, and said third means includes means for detecting said EOB codes, and responsive to detection of EOB codes segmenting respective
blocks of data.

4. In a video signal processing system for processing digital compressed video data conveyed as high and low priority data in high and low priority channels respectively, said high and low priority data being generated from encoded compressed
video data occurring as blocks of codewords, respective blocks including pluralities of codewords occurring in descending order of importance for image reproduction, and wherein data through a jth codeword type of respective blocks are allocated as high
priority data and the remaining codewords for respective blocks are allocated as low priority data, apparatus for combining said high priority data from respective blocks of data in said high priority channel with low priority data from corresponding
blocks of data from said low priority channel comprising:

means responsive to high priority data conveyed in said high priority channel for identifying respective blocks of high priority data;

means for appending codewords of low priority data from respective blocks to codewords from corresponding blocks of high priority data to form blocks of data representing respective image areas.

5. The apparatus set forth in claim 4 wherein at least a portion of said high priority data is variable length encoded and successive codewords are conveyed without delineation, a codeword number CW#j identifying said jth codeword type is
separably conveyed with said high priority data, and said means responsive to conveyed high priority data for identifying respective blocks of high priority data includes;

means for recovering said codeword CW#j;

decoding means for delineating said high priority codewords;

means, responsive to said codeword CW#j for segmenting said separated codewords into said respective blocks of high priority data.

6. The apparatus set forth in claim 4 wherein at least a portion of said low priority data is variable length encoded and successive codewords are conveyed without delineation, and the last codeword of each respective block is an end of block
code EOB, and wherein said means responsive to conveyed low priority data for identifying respective blocks of low priority data includes:

decoding means for recognizing said EOB code; and

means, responsive to said EOB code for segmenting said low priority codewords into said respective blocks of low priority data.

7. Apparatus for combining high and low priority video data conveyed in high and low priority data channels, said high and low priority data being derived from blocks of data, each block of data including a plurality of codewords, and a portion
of said plurality of codewords from respective blocks is selected as high priority data and the remainder of said plurality of codewords of respective blocks is allocated as low priority data and wherein an indicia is transmitted with said high priority
data which indicates the respective portions of respective blocks selected as high priority data, said apparatus comprising:

first means responsive to said high and low priority data for decoding said data and providing respective sequences of codewords of high and low priority data;

second means responsive to said high and low priority sequences of codewords and said indicia, for providing a plurality of codewords of said high priority sequence corresponding to a respective said portion of a respective block of data and
providing a plurality of codewords of said low priority sequence through a predetermined codeword of said low priority sequence, said plurality of codewords of said low priority sequence corresponding to the remainder of said plurality of codewords of
said respective block of data; and

means for combining said plurality of codewords of said low priority sequence with said plurality of codewords of said high priority sequence.

8. The apparatus set forth in claim 7 wherein said second predetermined codewords are end of block codes.

9. The apparatus set forth in claim 7 wherein said indicia conditions said second means to divide said sequence into portions at every jth codeword in at least a portion of said high priority sequence of codewords.

10. The apparatus set forth in claim 9 wherein said high and low priority data are variable length coded and said first means includes variable length decoding means.

11. The apparatus set forth in claim 7 wherein said high and low priority video data are conveyed in mutually exclusive high and low priority transport packets of predetermined numbers of bits including respective transport packet headers, said
transport packet headers including information for identifying segments of the respective high priority video data conveyed in respective high priority transport packets and identifying corresponding segments of the low priority video data conveyed in
respective low priority transport packets, and wherein said second means is responsive to said high and low priority transport packet header information for aligning corresponding segments of said high and low priority data.

12. The apparatus set forth in claim 7 wherein said high and low priority video data are derived from a sequence of blocks of codewords, with each block ending with an EOB code, said high priority video data including a first occurring number of
codewords in respective blocks and is conveyed with said indicia indicating said number, and said low priority video data consists of the remaining codewords in respective blocks and including said EOB code and wherein said second means is responsive to
said number for switching between providing high and low priority codewords and is responsive to said EOB codes for switching between providing low and high priority codewords.

13. In a television processing system for decoding compressed video signal transmitted as high and low priority data in high and low priority channels respectively, said data including codewords CW of differing types, said types representing a
hierarchy of greater to lesser importance to image reproduction with codeword types of greater and lesser importance being included in said high and low priority channels respectively, said codewords occurring in blocks of data, with data from respective
blocks occurring in both said high and low priority channels, and wherein the data of said high and low priority channels are conveyed in respective transport packets having header data including block identifiers, and the header data of the high
priority channel transport packets including codewords CW#j identifying types of codewords included in said high priority channel, apparatus for combining said data from said high and low priority channels into a single data stream comprising:

first means, responsive to received signal, for providing said high and low priority data with header data excised, and for providing said header data;

second means, responsive to said codewords CW#j in said header data, for segmenting said high priority data into respective partial blocks of data;

third means, responsive to said low priority data, for segmenting said low priority data into respective partial blocks of data; and

fourth means, responsive to said header data for combining corresponding partial blocks of high and low priority data to provide reconstructed blocks of data as said single data stream.

14. The system set forth in claim 13 wherein said low priority data provided in said low priority channel includes end of block codes, and said third means is responsive to said end of block codes for segmenting said low priority data into said
partial blocks.

15. The system set forth in claim 14 wherein said fourth means includes said end of block codes in the partial blocks of low priority data combined with said partial blocks of high priority data.

16. The system set forth in claim 13 wherein said transport blocks include error check codes, and said first means includes error detection circuitry, responsive to said error check codes for generating an error signal on the occurrence of
transmission errors in at least transport blocks of low priority data, and said fourth means includes means for substituting end of block codes for partial blocks of low priority data from transport blocks for which errors are detected.

17. The system set forth in claim 13 wherein said codewords are encoded as variable length codewords, and said second and third means include variable length decoding means responsive to said codewords, for establishing demarcation between
respective codewords.

Description

This invention relates to a system for processing a high definition television (HDTV) signal, and more particularly to apparatus for recombining transmitted compressed and
encoded data which has been prioritized and conveyed in high and low priority channels.

BACKGROUND OF THE INVENTION

Digital high definition video data may be successfully transmitted over terrestrial television channels, as described herein, by partitioning the video data between high and low priority information, and quadrature amplitude modulating the high
and low priority data on separate carriers respectively. The modulated carriers are included in a 6 MHZ frequency spectrum and then the combined signal is translated to occupy a standard broadcast channel spectrum. The high priority data is transmitted
with relatively high power and the low priority data with relatively low power. High priority data is that video data which is sufficient to reproduce an image, albeit of lesser quality than a high definition image.

The present invention is directed to circuitry for recombining, at the receiver, compressed video data which has been prioritized between relatively high and low priority video data. For purposes of this disclosure the video data will be
presumed to be compressed in MPEG like format. What is meant by "MPEG like" is a coding format similar to the standardized coding format being established by the International Organization for Standardization. The standard is described in the document
"International Organization for Standardization", ISO-IEC JT(1/SC2/WG1), Coding of Moving Pictures and Associated Audio, MPEG 90/176 Rev.2, Dec. 18, 1990, which document is incorporated herein by reference for description of the general code format.

In the system described herein the video signal is compressed in such fashion to develop blocks of data codewords. The data in respective blocks are divided between high and low priority transmission channels according to the relative importance
of the codewords for image reconstruction. The compressed data in the high and low priority channels include, at least in part, variable length encoded data without codeword demarcation.

At the receiver high and low priority data from corresponding blocks is recombined for decompression purposes. The fact that the transmitted data codewords do not have defined boundaries complicates the recombination of high and low priority
data.

SUMMARY OF THE INVENTION

The present invention relates to apparatus for receiving prioritized data from two channels and combining the data from the two channels into a common data stream. The signal operated upon is conveyed as high and low priority data in high and
low priority channels respectively, said high and low priority data being generated from encoded compressed video data occurring as blocks of codewords in descending order of importance for image reproduction, and wherein data through a jth codeword of
respective blocks are allocated as high priority data and the remaining codewords for respective blocks are allocated as low priority data. The apparatus includes a first circuit means, responsive to conveyed high priority data, for identifying
rspective blocks of high priority data; a second circuit means, responsive to conveyed low priority data, for identifying respective blocks of low priority data; and a third circuit means for appending respective blocks of codewords of low priority data
to corresponding blocks of codeowords of high priority data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagrams of an HDTV encoding/decoding system embodying the invention.

FIGS. 1B-1C are pictorial representations of sequences of field/frames of encoded video signal useful in describing the invention.

FIG. 2 is a pictorial representation of a macroblock of data provided by the compression circuit of FIG. 3.

FIG. 3 is a block diagram of a video signal compression circuit.

FIG. 3A is a generalized pictorial representation of the data format provided by the compression circuit of FIG. 3.

FIG. 4 is a block diagram of exemplary circuitry which may be utilized for the formatting circuit 111 of FIG. 3.

FIG. 5 is a block diagram of exemplary circuitry which may be implemented for the Priority Select circuitry of FIG. 1.

FIG. 5A is a flow diagram illustrating the operation of the FIG. 5 analyzer.

FIG. 6 is a diagram of the signal format provided by the transport processor 12 of FIG. 1.

FIG. 7 is a block diagram of exemplary circuitry which may be implemented for the Transport Process circuit of FIG. 1.

FIG. 8 is a block diagram of exemplary circuitry which may be implemented for the transport processor 25 of FIG. 1.

FIG. 9 is a block diagram of exemplary circuitry which may be implemented for the priority deselect circuit 26 of FIG. 1.

FIG. 10 is a block diagram of exemplary circuitry which may be implemented for the decompression circuit 27 of FIG. 1.

FIG. 10A is a block diagram of a combination of an alternative priority deselect circuit and a decompression circuit.

FIG. 10B is a flow chart of the operation of the FIG. 10A priority deselect circuit.

FIG. 11 is a block diagram of exemplary circuitry which may be implemented for the MODEMS 17 and 20 of FIG. 1.

DETAILED DESCRIPTION

An exemplary HDTV system which may be supported by the present invention includes a two-to-one interlaced signal of 1050 lines at 59.94 frames per second. The nominal active picture has 960 lines of 1440 pixels each with an aspect ratio of
16.times.9. The signal is transmitted using two 64 quadrature amplitude modulated (64-QAM) carriers, frequency multiplexed in a 6 NMHz transmission band. The nominal total bit rate, including video, audio and auxiliary data, is 26-29 Mbps.

The video signal is initially compressed in conformance with an MPEG-like format, albeit using both fields of each frame and with higher pixel density. Thereafter the MPEG-type signal codewords are parsed into two bit streams in accordance with
the relative importance of the respective codeword types. The two bit streams are independently processed to apply error correction overhead bits, and then caused to QAM respective carriers. The modulated carriers are combined for transmission. The
bit streams of relatively greater and lesser importance are designated high priority (HP) and low priority (LP) channels respectively. The high priority channel is transmitted with approximately twice the power as the lower priority channel. The high
priority/low priority information ratio is approximately one-to-four. The approximate net data rates after forward error correction are 4.5 Mbps HP and 18 Mbps LP.

FIG. 1 illustrates an exemplary HDTV encoding/decoding system according to the invention. FIG. 1 shows the system processing a single video input signal, but it is to be understood that the luminance and chrominance components are compressed
separately, and that the luminance motion vectors are utilized for generating compressed chrominance components. The compressed luminance and chrominance components are interleaved to form macroblocks before codeword priority parsing.

A sequence of image field/frames as per FIG. 1B is applied to circuitry 5 which reorders the field/frames according to FIG. 1C. The reordered sequence is applied to a compressor 10 which generates a compressed sequence of frames that are coded
according to an MPEG-like format. This format is hierarchical and is illustrated in abbreviated form in FIG. 3A.

The MPEG hierarchical format includes a plurality of layers each with respective header information. Nominally each header includes a start code, data related to the respective layer and provision for adding header extensions. Much of the
header information (as indicated in the referenced MPEG document) is required for synchronization purposes in an MPEG systems environment. For purposes of providing a compressed video signal for a digital HDTV simulcast system, only descriptive header
information is required, that is start codes and optional extensions may be excluded. The respective layers of the coded video signal are illustrated pictorially in FIG. 2.

When referring to the MPEG-like signal produced by the present system what is meant is that a) successive fields/frames of video signal are encoded according to an I, P, B coding sequence, and b) coded data at the picture level is encoded in
MPEG-like slices or group of blocks albeit that the number of slices per field/frame may differ and the number of macro blocks per slice may differ.

The coded output signal of the present system is segmented in groups of fields/frames (GOF) illustrated by the row of boxes L2 (FIG. 3A). Each GOF (L2) includes a header followed by segments of picture data. The GOF header includes data related
to the horizontal and vertical picture size, the aspect ratio, the field/frame rate, the bit rate, etc.

The picture data (L3) corresponding to respective fields/frames includes a header followed by slice data (L4). The picture header includes a field/frame number and a picture code type. Each slice (L4) includes a header followed by a plurality
of blocks of data MBi. The slice header includes a group number and a quantization parameter.

Each block MBi (L5) represents a macroblock and includes a header followed by motion vectors and coded coefficients. The MBi headers include a macroblock address, a macroblock type and a quantization parameter. The coded coefficients are
illustrated in layer L6. Note each macroblock is comprised of 6 blocks, including four luminance blocks, one U chrominance block and one V chrominance block. See FIG. 2. A block represents a matrix of pixels. e.g., 8.times.8 over which a discrete
cosine transform (DCT) is performed. The four luminance blocks are a 2.times.2 matrix of contiguous luminance blocks representing, e.g., a 16.times.16 pixel matrix. The chrominance (U and V) blocks represent the same total area as the four luminance
blocks. That is the chrominance signal is subsampled by a factor of two horizontally and vertically relative to luminance, before compression. A slice of data corresponds to data representing a rectangular portion of an image corresponding to an area
represented by a contigous group of macroblocks.

The block coefficients are provided one block at a time with the DCT, DC coefficient occurring first followed by respective DCT AC coefficients in the order of their relative importance. An end of block code EOB is appended at the end of each
successively occurring block of data.

The amount of data provided by the compressor 10 is determined by the rate control element 18. As is well known compressed video data occurs at variable rates, and desirably, data is transmitted at a constant rate equivalent to the channel
capacity, to realize efficient use of the channel. Rate buffers 13 and 14 perform the variable to constant data rate translation. It is also known to adjust the amount of data provided by the compressor in accordance with the level of occupancy of the
buffers. Thus the buffers 13 and 14 include circuitry to indicate their respective level of occupancy. These indications are applied to the rate controller 18 to adjust the average data rate provided by the compressor 10. The adjustment is typically
accomplished by adjusting the quantization applied to the DCT coefficients. Quantization levels may be different for the different types of frame compression. Details of an exemplary method for determining quantization levels may be found in
application Ser. No. 494,098, filed Mar. 15, 1990 and entitled "Digital Signal Coding With Quantization Level Computations," which application is included herein by reference.

Compressed video data hierarchically formated as indicated in FIG. 3A is coupled to a priority select element 11, which parses the coded data between a high priority channel HP and a low priority channel LP. High priority information is that
information, the loss or corruption of which, would create the greatest degradation in reproduced images. Stated conversely, it is the least data needed to create an image, albeit less than a perfect image. Low priority information is the remaining
information. The high priority information includes substantially all of the header information included in the different hierarchical levels plus the DC coefficients of the respective blocks and a portion of the AC coefficients of the respective blocks
(level 6, FIG. 3A).

The ratio of HP and LP data at the transmitter is approximately 1:4. At the transport processor auxiliary data is added to the signal to be transmitted. This auxiliary signal may include digital audio signal and, e.g., teletext data. In this
example at least the digital audio will be included in the HP channel. The average amount of the auxiliary data included in the HP channel is calculated and compared to the expected statistical average of the compressed video information. From this the
ratio of high and low priority compressed video information is calculated. The priority select element parses the data provided by the compressor 10 according to this ratio.

The HP and LP compressed video data are coupled to a transport processor 12 which a) segments the HP and LP data streams into transport blocks, b) performs a parity or cyclic redundancy check on each transport block and appends the appropriate
parity check bits thereto, and c) multiplexes the auxiliary data with the HP or LP video data. The parity check bits are utilized by the receiver for isolating errors in conjunction with synchronizing header information and for providing error
concealment in the event of uncorrectable bit errors in the received data. Each transport block includes a header including information indicative of the type of information included in the block, e.g., video audio and pointers to the starting points of
contiguous like data.

The HP and LP data streams from the transport processor 12 are applied to the respective rate buffers 13 and 14, which convert the variable rate compressed video data from the processor 12 to data occurring at a substantially constant rate. The
rate adjusted HP and LP data are coupled to forward error encoding elements 15 and 16 which a) perform REED SOLOMON forward error correction encoding independently to the respective data streams; b) interleave blocks of data to preclude large error
bursts from corrupting a large contiguous area of a reproduced image; and c) appends, e.g., Barker codes to the data for synchronizing the data stream at the receiver. Thereafter the signals are coupled to a transmission modem wherein the HP channel
data quadrature amplitude modulates a first carrier and the LP channel data quadrature amplitude modulates a second carrier displaced from the first carrier by approximately 2.88 MHz. The 6 dB bandwidth of the modulated first and second carriers are
respectively about 0.96 MHz and 3.84 MHz. The modulated first carrier is transmitted with approximately 9 dB greater power than the modulated second carrier. Since the HP information is transmitted with greater power it is much less prone to corruption
by the transmission channel. The HP carrier is located in the portion of the frequency spectrum of an, e.g., NTSC TV, transmission channel normally occupied by the vestigial sideband of a standard NTSC TV signal. This portion of the signal channel is
normally significantly attenuated by the Nyquist filters of standard receivers and thus HDTV signals with this transmission format will not introduce cochannel interference.

At the receiver the transmitted signal is detected by the modem 20 which provides two signals corresponding to the HP and LP channels. These two signals are applied to respective REED SOLOMON error correcting decoders 21 and 22. The error
corrected signals are coupled to rate buffers 23 and 24 which receive data at a variable rate commensurate with the requirements of the subsequent decompression circuitry. The variable rate HP and LP data is applied to a transport processor 25 which
performs the inverse process of the processor 12. In addition it performs a degree of error detection responsive to the parity check bits included in the respective transport blocks. The transport processor 25 provides separated auxiliary data, HP
data, LP data and an error signal E. The latter three signals are coupled to a priority deselect processor 26 which reformats the HP and LP data into an hierarchically layered signal which is applied to a decompressor 27. The decompressor 27 performs
the inverse function of the compressor 27.

FIG. 3 illustrates an exemplary compressor apparatus which may be utilized for element 10 of FIG. 1, for providing hierarchically layered compressed video data. The apparatus shown only includes the circuitry required to generate compressed
luminance data. Similar apparatus is required to generate compressed chrominance U and V data. In FIG. 3 there are elements 104 and 105 designated as elements for computing forward and backward motion vectors respectively. Since whether a motion
vector is forward or backward depends only upon whether the current field is analyzed with respect to a prior or succeeding field, both elements are realized with similar circuitry, and in fact both elements 104 and 105 alternate on a field/frame basis
between generating forward and backward vectors. The elements 104 and 105 may be realized using integrated circuits of the type designated STI 3220 MOTION ESTIMATION PROCESSOR available from SGS-THOMSON MICROELECTRONICS. In order to achieve the
necessary processing rates each of the elements 104 and 105 comprise a plurality of such integrated circuits operating simultaneously on different areas of respective images.

Element 109 designated DCT & Quantize performs the descrete cosine transform and quantization of transform coefficients and may be realized using integrated circuits of the type designated STV 3200 DISCRETE COSINE TRANSFORM available from
SGS-THOMSON MICROELECTRONICS. Element 109 will also be realized with a plurality of such devices operated in parallel to concurrently process different areas of the image.

Refer to FIG. 1C and assume that frame 16 is currently available. Previously occurring P frame 13 has been snatched and stored in the buffer memory B 101. In addition a generated predicted frame 13 has been stored in one of the buffer storage
elements 114 or 115. As frame 16 occurs it is stored in buffer memory A, 102. In addition frame 16 is applied to a working buffer memory 100. As frame 16 occurs, appropriate image blocks of data are coupled from the memory 100 to the minuend input of
a subtracter 108. During I frame compression the subtrahend input of the subtracter 108 is held at a zero value so that data passes through the subtracter 108 unaltered. This data is applied to the DCT and quantizer element 109 which provides quantized
transform coefficients to elements 110 and 112. Element 112 performs inverse quantization and inverse DCT transformation of the coefficients to generate a reconstructed image. The reconstructed image is applied via an adder 113 to, and stored in, one
of the buffer storage elements 114 and 115 for use in compressing subsequent B and P frames. During compression of I frames no information is added (by adder 113) to the reconstructed image data provided by element 112.

Element 110 performs two functions during I frame compression. First it performs differential (DPCM) coding of the DC coefficients generated by element 109. It then variable length encodes (VLC) the differentially coded DC coefficients and zero
run and variable length encodes the AC coefficients generated by element 109. The VLC codewords are applied to a formatter 111 which segments the data and appends header information thereto in conformance with the layers illustrated in FIG. 3A. Coded
data from element 111 is then passed to the priority select apparatus. Each of the elements 109, 110 and 111 are controlled by a system controller 116 to cyclically perform the appropriate operations at the appropriate times.

After frame 16 a "B" frame (14) occurs and is loaded into buffer memory 100. Data from frame 14 is coupled to both of elements 104 and 105. Element 104, responsive to frame 14 data from memory 100 and frame 13 data from memory 101, calculates
forward motion vectors for respective blocks of 16.times.16 pixels of image data. It also provides a distortion signal which is indicative of the relative accuracy of the respective forward motion vectors. The forward motion vectors and the
corresponding distortion signals are coupled to an analyzer 106.

Element 105, responsive to frame 14 data from memory 100 and I frame 16 data from memory 102, generates backward motion vectors and corresponding distortion signals which are also coupled to the analyzer 106. Analyzer 106 compares the distortion
signals against a threshold, and if both exceed the threshold, provides both the forward and backward motion vectors as the motion vector, and also provides a corresponding signal related to the ratio of the distortion signals. Upon reconstruction
predicted images are generated using both forward and backward vectors and corresponding frame data from which derived. An interpolated frame is generated from the forward and backward predicted frames in accordance with the ratio of distortion signals. If the distortion signals for both the forward and backward motion vectors are less than the threshold, the motion vector with the corresponding lesser valued distortion signal is selected as the block motion vector.

After the motion vector has been determined, it is applied to the motion compensated predictor 107 which accesses the appropriate data block defined by the vector from the previously regenerated frame 16 or frame 13 or both, stored in the storage
elements 114 and 115. This data block is applied to the subtrahend input of the subtracter 108 wherein it is subtracted on a pixel by pixel basis from the corresponding block of pixel data from the current frame 14 provided by the buffer memory 100.
The differences or residues are then encoded in element 109 and the coefficients applied to element 110. The corresponding block vector is also applied to element 110. For encoded B and P frames, the DC coefficients are not differentially encoded, but
both the DC and AC coefficients are variable length encoded. The motion vectors are differentially encoded and then the differentially encoded vectors are variable length encoded. The coded vectors and coefficients are then transferred to the formatter
111. The encoded B frames are not inverse quantized and inverse transformed in element 112 since they are not used for subsequent encoding.

P frames are similarly encoded except that only forward motion vectors are generated. For example P frame 19 is encoded with motion vectors associating corresponding blocks of I frame 16 and P frame 19. During encoding of P frames, element 112
provides corresponding decoded residues and element 107 provides the corresponding predicted P frame. The predicted frame and the residues are added in adder 113 on a pixel-by-pixel basis to generate the reconstructed frame which is stored in the one of
storage elements 114 and 116 not containing the frame information from which the predicted P frame is generated. The reconstructed and stored P frame is used for encoding subsequent B frames. For both P and B field/frames it should be noted that DCT's
are performed on a block basis (e.g., a matrix of 8.times.8 pixels), but motion vectors are calculated for macroblocks (e.g., a 2.times.2 matrix of luminance of blocks or a 16.times.16 matix of pixels).

FIG. 4 shows exemplary circuitry in block form which may be used to implement the functions of elements 110 and 111 of FIG. 3. The output format of this circuitry diverges from that normally provided by an MPEG type encoder in that an MPEG
output is a bit-serial data stream, but the data provided by the exemplary circuitry of FIG. 4 is in parallel-bit word format. This format is selected to facilitate the implementation of both the priority select processor and the transport processor.
In addition two extra signals are provided which define the code type of each output codeword, CW, and the length, CL, of each codeword.

In FIG. 4 motion vectors from the analyzer 106 (FIG. 3) are differentially encoded in the DPCM element 127 on a slice basis and coupled to a multiplexer 129 via a buffer memory 133. Transform coefficients from the transform element 109 are
coupled to a multiplexer 132 and a differential encoding element DPCM 128. Differentially encoded coefficients from the DPCM 128 are coupled to a second input of the multiplexer 132. During encoding of P or B frames all of the coefficients are passed
directly through the multiplexer 132. During encoding of I frames, the DC coefficients are selectively differentially encoded by the DCPM 128. The differential encoded DC coefficients and the non-differentially encoded AC coefficients are multiplexed
by the multiplexer 132 and coupled to a second input of the multiplexer 129 via the buffer memory 133. Header information from a format control and header element 126 are coupled to a third input of the multiplexer 129. Element 126 includes stored
information and control circuitry to a) provide the requisite header information for the different code layers (FIG. 3A) and to b) provide control signals to time division multiplex the header information, motion vectors and the transform coefficients
via multiplexer 129. Element 126 is responsive to the system control circuitry via the control bus CB to provide the appropriate headers corresponding to picture size, rate, picture coding type quantizer parameters, etc. Certain of the header
information is calculated by element 126 in conjunction with an analyzer 125. In the MPEG-type format much of the header information (e.g., level 5 of FIG. 3A) is variable such as the type of block encoding, the type of motion vectors, whether a block
has zero valued motion vectors and or whether all coefficients in a block are zero values. The vector information and coefficient information is applied to the analyzer 125 to determine these types of header information. Whether a motion vector is a
forward, backward and or zero valued is directly determinable by examination of the vectors. Whether all coefficients in a block are zero valued is determinable by simply accumulating the magnitudes of the vectors included in a block. Once the type of
variable header data is determined, it is assigned a codeword and provided to the multiplexer 129 at the appropriate time. The element 126 also provides information relating to the codeword type currently being multiplexed, i.e., header information,
motion vector information, DC coefficients, AC coefficients.

The time division multiplexed information is coupled to a variable length encoder 130 which is also controlled by element 126. In the figure the VLC control is shown provided by the codeword type signal. The different code types are variable
length coded according to different VLC code tables and thus it is appropriate to utilize the code type signal for such control.

The VLC 130 may include a zero-run encoder for encoding zero runs of the AC coefficients and a plurality of Huffman code tables addressed by the respective codewords passed by the multiplexer 129 for variable length encoding the transform
coefficients and the motion vectors. The particular table utilized is enabled by the code type signal. Each of the code tables may include corresponding tables programmed with the code lengths of the respective variable length codewords. The codewords
CW and the code lengths CL are provided concurrently on separate busses in parallel-bit format. In general the header information is not variable length coded and is passed unaltered by the VLC 130. However, the VLC 130 includes code length tables
responsive to the code type signal to provide the code lengths of the header codewords. Alternatively, a bit counter may be included in the VLC to count the number of bits of these data.

The element 126 also controls the writing and reading of data provided to and from the buffer memory 133.

FIG. 5 illustrates exemplary apparatus for performing the priority selection process. This apparatus may operate in several modes. For example, data may be prioritized on an equal basis for the different field/frame types, or on an unequal
basis for the different field/frame types. In the latter instance assume that the HP channel passes 20 percent of the total data transmitted and that three percent of the HP channel is consumed by auxiliary data. If the video data is quantized for
maximum transmission channel efficiency, 17.53% of the video data may be allocated to the HP channel. In the former instance, the high priority data for the I, P and B frames may be assigned in for example, the ratio of .alpha.:.beta.:1 respectively.
The values .alpha. and .beta. may be user selectable and/or determined on a statistical basis from the amount of code data from prior encoded frames.

Refer to FIGS. 5 and 5A. In the following description, numbers in square brackets correspond to process blocks of FIG. 5A. Data from the variable length encoder 130 is coupled to respective input ports of two buffer memories 150A and 150B and
to a data analyzer 152. The respective buffers include enough memory to store for example a slice of data. The buffers 150A and 150B are operated in "ping-pong" fashion to alternately write slices of data and read slices of data. Thus while buffer
150A writes data from, e.g., slice n, buffer 150B reads data from slice n-1.

As data is written to a particular buffer, the analyzer 152 generates a codeword number CW#i for each codeword and stores the CW#i in association with the corresponding codeword. The analyzer also calculates the point, or codeword, at which the
data should be split between HP and LP channels. The calculation is determined for the amount of data stored in the buffer. There are four general types of data including Header Data, Motion Vectors, DC coefficients and AC coefficients. The DC and AC
coefficients over a block occur in the order of DC coefficient first followed by codewords representing the AC coefficients in generally descending order of importance. The total number of bits are counted for all the codewords in the buffer. Then the
codeword at which the sum of bits is just greater than the HP percentage is identified by a codeword number, CW#j. This number is applied to a switching element 153A (153B), and used to control the multiplexer 155A (155B). After the codeword number CW#j
is identified, the codewords, codelength data, codeword type data and codeword numbers are read in parallel from the buffer 150A (150B). The codewords, codelengths and codetypes are applied to the input of a multiplexer 155A (155B), and the codeword
numbers are applied to an input of the switching element 153A (153B). As the data is read from the buffer, the switching element 153A (153B) compares the codeword numbers to the calculated number CW#j. For all codeword numbers less than or equal to
CW#j, the switching element provides a control signal which conditions the multiplexer 155A (155B) to pass the corresponding data to the HP channel via a further multiplexer 156. For codeword numbers greater than CW#j, the multiplexer 155A (155B) is
conditioned to pass the corresponding data to the LP channel via multiplexer 156. The multiplexer 156 is conditioned to pass HP and LP data provided by the buffer 150A, (150B) which is currently being read.

The analyzer 152 is responsive to the codelength signals, and the code type signals. Responsive to the code type signals, the analyzer generates [502] codeword numbers for each occurring codeword. For example, each codeword representing header
information is assigned the number (-2). Each codeword representing motion vectors and DC coefficients are assigned the numbers (-1) and (0) respectively. Successive AC codewords are assigned ascending integers i from 1 to n on a block-by-block basic.

The analyzer 152 also includes an accumulator which, responsive to the codelength and type signals, independently sums the number of bits of the codewords of each code type entered in the buffer 150A (150B). These sums are added [504] to provide
the total number of codeword bits contained in the buffer. The total sum is multiplied by the decimal equivalent of the percent to be allotted to the HP channel to produce a check sum [512]. Thereafter the respective code type sums are sequentially
added [508] in ascending order of codeword number CW#i to produce partial sums. Each partial sum is compared [512] with the check sum until the partial sum exceeds the check sum. The codeword number CW#j associated with the immediately previous partial
sum is the last codeword within a block to be assigned to the HP channel [512-518]. All succeeding codewords, i.e., CW#j+1 to CW#n, for respective blocks are assigned to the LP channel.

Respective HP and LP data from the priority selector is arranged in transport blocks designed to enhance signal recovery and error concealment at the receiver. The transport block format is illustrated in FIG. 6. An exemplary HP transport block
includes 1728 bits and an LP transport block includes 864 bits. Respective transport blocks may include more or less than a slice of data. Thus a particular transport block may include data from the end of one slice and data from the beginning of the
next subsequent slice. Transport blocks including video data may be interleaved with transport blocks containing other data, e.g., audio. Each transport block includes a service type header ST which indicates the type of information included in the
respective transport block. In this example the ST header is an 8-bit word which indicates whether the data is HP or LP, and whether the information is audio, video or auxiliary data. Four bits of the 8-bit word are used to represent the ST information
and four bits are used to provide Hamming parity protection of the ST information bits.

Each transport block includes a transport header TH immediately following the ST header. For the LP channel the transport header includes a 7-bit macroblock pointer, an 18-bit identifier and a 7-bit record header (RH) pointer. The transport
header of the HP channel includes only an 8-bit record header (RH) pointer. The macroblock pointer is used for segmented macroblock or record header components, and points to the start of the next decodable component. For example, if the particular
transport block includes macroblock data associated with the end of slice n and the beginning of slice n+1, the data from slice n is placed adjacent the transport header and the pointer indicates that the next decodable data is adjacent the transport
header TH. Conversely, if a record header RH is adjacent the TH, the first pointer indicates the byte position following the record header RH. A zero valued macroblock pointer indicates that the transport block has no macroblock entry point.

The transport block may include none, one or more than one record header, and their positions are variable within the transport block. A record header occurs at the beginning of each slice of macroblock data in the HP and LP channel. No record
headers are included in transport blocks that include only video data header information. The record header (RH) pointer points to the byte position containing the start of the first record header in the transport block. Note, the first record header
in a transport block is placed at a byte boundary. That is, if a variable length code precedes the record header, the variable length code may be bit-stuffed to insure that the start of the record header occurs at a bit position which is an integral
number of bytes from the beginning of the transport block. The record headers are placed at byte boundaries to enable the decoder to locate them since they are embedded in a stream of concatenated variable length codewords. A zero valued RH pointer
indicates that there are no record headers in the transport block. If both the record header pointer and the macroblock pointer are zero valued, this state indicates that the transport block includes only video data header information.

The 18-bit identifier in the LP transport header identifies the current frame type, the frame number (modulo 32), the current slice number, and the first macroblock contained in the transport block.

Following the transport header is either a record header, RH, or data. As indicated in FIG. 6 the record header for the video data in the HP channel includes the following information: A 1-bit FLAG which indicates if a header extension, EXTEND,
is present. Following the FLAG is an identifier IDENTITY, which indicates a) the field/frame type I, B or P; b) a field/frame number (modulo 32) FRAME ID; and c) a slice number (modulo 64) SLICE IDENTITY. Following the identifier the record header
includes a macroblock priority break point indicator, PRI BREAK (j). The PRI BREAK(j) indicates the codeword number CW#j, developed by the analyzer 152 of the priority selector, for dividing the codewords between the HP and LP channels. Lastly, an
optional header extension may be included in the HP record header.

The record header incorporated in the LP channel includes only an identifier, IDENTITY, similar to the identifier implemented in the HP channel.

Each transport block is terminated with a 16-bit frame check sequence, FCS, which is calculated over all bits in the transport block. The FCS may be generated using a cyclic redundancy code.

FIG. 7 illustrates exemplary apparatus of the transport processor. In the figure an arbiter 213 interleaves, via a multiplexor 212, transport blocks of video data from a multiplexer 211, audio data from a memory 214 and auxiliary data from a
memory 215. The audio data is provided in transport block form by the source 216 and applied to a first-in first-out memory 214. The auxiliary data is provided in transport block form by the source 217 to a first-in first-out memory 215. The formats
of the audio and auxiliary data transport blocks may differ from the format of the video transport blocks, however all transport blocks will include a leading service type header, and preferably will be of equal length. The arbiter 213 responds to the
level of occupancy of buffers 214, 215 and 207 in such fashion to insure that none of these buffers overflow.

The apparatus of FIG. 7 operates on one of the HP or LP signals and similar apparatus is required for the alternate signal. However, if all of the audio and auxiliary signal is HP data an arbiter for interleaving transport blocks will not be
included in the LP transport block processor and vice versa.

In FIG. 7, codeword, CW, codelength, CL, and code type, TYPE, data from the priority selector are coupled to a transport controller 218, and the codewords and code type signal are coupled to a variable word length to fixed word length convertor
201. The convertor 201 packetizes the variable length codewords into, for examples, 8-bit bytes in order to reduce the amount of storage space required of the rate buffers 13 and 14. The convertor 201 may be of the type described in U.S. Pat. No.
4,914,675. The fixed length words provided by the convertor 201 are temporarily stored in the buffer 207.

The transport controller 218 responds to the CW, CL, TYPE and CW#j data to construct the transport block headers (ST, TH, RH) and applies these headers to a header buffer 208, which may be internal to the controller 218. Responsive to the code
lengths, code types and codewords, the controller 218 generates the requisite timing signals for interleaving (via the multiplexer 209) the fixed length video data words and transport block header information into transport blocks of predetermined
numbers of bits.

The transport blocks provided by the multiplexer 209 are coupled to one input of the multiplexor 211 and to the input terminal of a frame check sequence coder FCS 210, the output of which is coupled to a second input of the multiplexer 211. The
FCS 210, responsive to transport block data forms two-byte error check codes for respective transport blocks. The multiplexer 211 is conditioned to pass respective transport blocks provided by the multiplexer 209, and then to append the 16-bit or
two-byte FSC code from element 210 to the end of the transport block.

In the foregoing description of the transport processor it is assumed that all of the header information provided by the compressor 10 is included in the video data stream provided by the transport processor. It should be recognized that much of
the video data header information is also included in the transport headers and as such provides redundant information. In an alternative arrangement, the controller 218 may preclude the converter 201 from accepting video header data which would be
redundantly included in the transport block headers, thus enhancing the overall coding efficiency. At the receiver the excised video header data may be reconstructed from the transport block header information and reinserted in the video data stream.

At the receiver, detected signal is applied to forward error correction circuits 21 and 22 to perform error correction on the respective HP and LP signals. Error corrected data is then applied to the transport processor 25 via rate buffers 23
and 24. Even though the detected data has undergone error correction in the FEC circuits 21 and 22, certain errors occurring during signal transmission may not be correctable by the FEC circuits. If these errors are allowed to pass to the decompression
circuitry, very objectionable corruption may occur in the reproduced image. To preclude such happenstance each transport block includes independent error detection codes to identify the occurrence of errors which pass through the FEC circuits, and
responsive to such error indications the system may provide appropriate error concealment.

FIG. 8 illustrates the transport processor 25 included in the receiver portion of the system. Two such processors are required, one for the HP channel and one for the LP channel. If it is known a priori that audio or auxiliary data will always
be excluded from a particular channel, the corresponding elements may be eliminated from such channel transport processor.

In FIG. 8 data from the rate buffer 23 or 24 is applied to an FCS ERROR detector 250 and a delay element 251. Delay element 251 provides a delay of one transport block interval to allow the detector 250 to determine if any errors are present in
the corresponding transport block. The detector 250 provides an error signal E indicating the presence or absence of errors in the transport block. The error signal is applied to the input port of a one-to-three demultiplexer 253. The delayed
transport block data is also applied to the input port of the multiplexer 253. Delayed transport block data is also coupled to a service type detector (ST DETECT) 252, which examines the ST header and responsive thereto conditions the multiplexer 253 to
pass the transport block data and corresponding error signal to the appropriate one of the audio, auxiliary or video signal processing paths. Even though an error may be detected in a transport block, the ST code may still be relied upon because it has
been independently Hamming code protected.

In the respective audio, auxiliary and video signal processing paths the error signal may be utilized in different ways to effect error concealment. In the video signal processing path the error signal may be utilized in alternative ways
depending upon the error concealment circuitry included in the decompressor 27. In the simplest case assume that the decompressor 27 includes a display memory in which information is updated as information is decoded. If no information is received for
a particular portion of an image, the corresponding portion of the display memory is not updated. Those image portions that are not updated are simply repeated in successive frames until new data is received. If it is assumed that error concealment by
repeating informtion from frame-to-frame is acceptable, in the video signal processing path the error signal may be utilized to simply excise transport blocks with detected errors from the video data stream. Alternatively, for more sophisticated error
concealment the transport block data may be retained but tagged with the error indication to alert the decompressor to perform alternative error concealment functions.

In the video signal processing path, the transport block data and error signal are coupled to a processing element 256 which excises the FSC code and the transport block headers ST, TH and RH from the data stream. It may also be arranged to
delete entire transport blocks in which errors have been detected. Element 256 provides the video data with transport block headers excised, the error data and the transport headers to the priority deselect processor 26 on separate busses.

The FEC circuits 21 and 22 provide the received data in fixed length words corresponding to the fixed length words provided to the FEC circuits 15 and 16 of the encoder. As such the transport block header data occurs on byte boundaries which are
either predetermined (ST, TH and FCS) or are identified (RH) by the transport header. Thus it is a relatively simple matter to identify and extract the requisite transport block headers from respective transport blocks.

FIG. 9 illustrates and exemplary priority deselect processor. The priority deselect processor accepts data from the receiver transport processor and reconfigures it to the form applied to the encoding priority select processor 11. In order to
do so respective codewords of the data stream have to be identified, that is the codeword CW#j in each block must be detectable. Since the data is in the form of concatenated variable length codes, it must be at least partially VLC decoded to define the
codeword boundaries. Once the codeword boundaries are determined, the codewords may be counted to find CW#j (in the HP channel). After the codeword boundaries are identified the codewords may easily be parsed into respective parallel-bit VLC code form.

In FIG. 9 similar circuits (270, 271, 272, 273) and (276, 277, 278, 279) responsive to HP and LP data respectively, parse the incoming data into parallel-bit VLC codewords. The HP and LP codewords are respectively coupled to a multiplexer 274
which, responsive to a deselect controller 275 recombines the data into a data sequence similar to that provided by the compressor 10.

Consider the HP channel circuitry 270-273. Video data from the element 256 (FIG. 8) is applied to a buffer memory 270 and a deselect controller 271. In addition the HP transport block headers are applied to the controller 271. The video data,
absent errors, will occur in predetermined cyclical sequences. Particular points in a sequence are indentifiable from the transport block header information. Once a start point is identified, decoding proceeds in the predetermined sequence. The
deselect controller 271 is programmed to condition the variable length decoder VLD for operation according to this sequence. For example, assume that the transport header indicates that current data is from an I field and that a record header occurred
at byte z. The record header was placed at the beginning of a slice, thus the entry point of a slice can be identified relative to byte z. At this point the slice header of known bit/byte coding format is known, which header is followed by a macroblock
header of known bit/byte coding format, followed by block data in known coding format, and so on. Thus, responsive to the transport header information the controller 271 establishes the decoding sequence of the VLD 272, i.e., which VLD decoding tables
to use for which grouping of VLC codewords. Note that since, e.g., the slice header in the data stream is not variable length encoded, the controller may be arranged to compare common slice header information with transport header information for entry
point confirmation.

The video data from the buffer 270 is coupled to the VLD 272 which concatenates a number of the fixed length codewords, and examines the leading bits of the concatenated codewords for a recognizable codeword according to the type of coding
expected vis-a-vis the normal cyclical sequence. Once a particular number of leading bits is recognized as a valid codeword, these bits are output as a parallel-bit codeword CW to a buffer store 273. In addition the expected codeword type, T, and
codeword length CL are produced and applied to the buffer store 273. As the codewords are loaded into the buffer store 273 they are indexed by the deselect controller 275.

DC and AC coefficient codewords are encoded according to different statistics, and the coefficients of respective blocks within a macroblock are concatenated without including end of block identifiers. In general however the DC coefficient of
the first block in a macroblock is identifiable by its position within the bit stream. The VLD cannot discern between the last AC coefficient of one block and the DC coefficient of the next block. Identification is provided by the CW#j included in the
transport block header information. The CW#j identifies the last AC coefficient codeword in each block in a slice. To find the codeword numbered (j) the deselect controller 275 monitors the code types T provided by the VLD. The controller 275 counts
the AC type codes T, and when j have occurred the controller 275 communicates with the VLD 272 to reset the cycle to a DC coefficient decoding event.

Elements 276-279 in the LP channel operate in a similar manner. The LP data however is expected to include only AC coefficient codewords. The AC codewords for respective blocks within a macroblock are separated by end of block (EOB) codes,
hence there is no need to count codewords. The operation of VLD 278 may simply be to decode words which are all coded according to one coding table. The position of the first macroblock in transport block is identified by the respective transport
header, and each successive macroblock is identified by record headers. This information is evaluated by the deselect controller 277 for controlling the VLD 278 for indexing the codewords in the storage buffer 279.

Responsive to the indexed information and the type T codewords stored in the storage buffers 273 and 279, the deselect controller 275 concatenates the HP and LP codewords stored in the storage buffers 273 and 274 via a multiplexer 274. The
controller identifies a macroblock, conditions the multiplexer 274 to pass data from the HP channel and reads the respective HP data from storage buffer 273 up to codeword CW#j of block one of the macroblock. It then conditions the multiplexer to pass
data from the LP channel and reads the AC coefficient codewords corresponding to the same block one, until an EOB type code occurs. Thereafter controller 275 conditions the multiplexer 274 to pass data from the HP channel and commences reading HP data
corresponding to block two of the macroblock. After the codeword corresponding to CW#j is read, the controller again switches to read LP data for block for block two from the LP channel, etc.

If while reading data from the high priority channel an EOB code occurs before the codeword corresponding to CW#j occurs, the controller 275 is reset to read the next block of data from the high priority channel.

The cyclic nature of occurring codewords may be variable. For example some macroblocks within a slice may not be coded and/or some blocks within a macroblock may not be coded. This information is included in the respective slice and macroblock
headers. In order to establish and maintain the appropriate decoding cycles, the controller 275, responsive to the codeword types examines the slice and macroblock header codewords to determine the numbers of blocks in respective macroblocks and the
number of marcoblocks in respective slices. Responsive to these numbers the controller 275 counts particular decoding operations and determines when certain decoding functions have been completed, and restarts a decoding cycle. Note that as indicated
previously more than one record header may be included in a transport block, but only the first record header is identified by the transport block header. As such only the first record header in a transport block may be excised by the element 256. In
order to identify and extract the information from such record headers, and excise such record headers from the data stream, the controller 275 counts the number of macroblocks processed by the VLD 272, and on completion of the last macroblock in a slice
recognizes the next occurring data in a transport block as a record header. It thereafter reads the information in the record header, to establish subsequent cyclic operations, and precludes its passage to the buffer 273.

The controllers 271, 275 and 277 are indicated in the figure as three separate elements, however it should be appreciated that they may be subsumed in a single controller element.

The circuitry of FIG. 9 does not provide variable length decoded data but rather only parses respective variable length codewords, and provides them in a form similar to the data provided at the output of the compressor 10. As such circuitry
substantially complementary to compressor 10 may be utilized for the decompressor 27. It should be recognized however that the circuitry of FIG. 9 may be arranged to provide decoded variable length codes obviating a VLD in the decompressor circuitry.

In the FIG. 9 provision is made for a variety of error concealment methodologies. For example, even though a transport block includes an error, the data for that block may be processed and passed on to the decompressor. In this instance an
error flag is generated for each data word of the transport block and carried along with the codewords applied to the decompressor. The error flags are provided by the deselect controllers 271 and 277 and coupled to the buffer stores 273 and 279,
wherein they are stored in memory locations corresponding with the associated erroneous transport block codewords.

In an alternative system wherein corrupted transport blocks are not processed, assume that a LP transport block is lost. The LP channel provides coefficient data of lesser importance to the reconstruction of images, and in fact DCT blocks may be
decompressed without these coefficients, albeit the respective decompressed blocks will exhibit less spatial resolution. Therefore, when LP erroneous transport blocks are excised from the data stream, and data is being reconstructed at the multiplexer
274, after each block codeword CW#j of HP data, an EOB code is inserted in liew of LP data. The EOB code is provided by the controller 275 and multiplexed into the data stream via the multiplexer 274. To indicate that the EOB for the respective blocks
is a forced or artificial EOB, an error flag may be carried along with the EOB signal. The forced EOB signal is designated EOBE.

The controller 275 is provided the transport header information for both channels and indexes the block information available in the storage buffers 273 and 279. The macroblock and block data occur in a known sequence, enabling the controller to
recognize lost data, and to provide and append EOBE codes to the HP data for lost LP data.

In general very few errors are expected to occur in the HP channel due to the robustness by which it is transmitted. However, if an error does occur in the HP channel, the data in the LP channel corresponding to blocks of data lost in the HP
channel becomes meaningless. The controller 275 is programmed to recognize lost HP data via an interruption of the normal sequence of information identified by the nonerroneous transport block headers. When lost HP data is detected, the corresponding
LP data is flushed from the storage buffer 279, that is, it is not passed on to the decompressor. In addition, the controller 275 may be arranged to provide error data to the decompressor, in a form which identifies lost information, i.e., macroblock or
slice or frame data which are not being provided from the priority deselect processor.

The controller 275 is responsive to the overall system controller via the control bus CB, to initialize or reinitialize the controllers 271, 277 and VLD's 272, 278 on start up and channel changes, etc. In addition the controller 275 communicates
with the transport processor 25 and rate buffers 23 and 24 for controlling the rate of information provided to the priority deselect circuitry.

Error concealment will not be discussed with respect to this apparatus as it is not the subject of this invention. Suffice it to say that error data from the priority deselect processor is applied to the decompression controller 302 to preclude
updating areas of the video display RAM 318 corresponding to missing blocks of data, and that the apparatus responds to forced EOB codes as if they are normally occurring EOB codes.

Generally the circuitry of FIG. 10 is arranged to decompress video data provided in MPEG-like hierarchical format. The video data provided by the multiplexer 274 of the priority deselect processor is applied to a buffer memory 300. This data is
accessed by the decompression controller 302 wherein header data is extracted to program the controller 302. The variable length codewords corresponding to DCT coefficients are extracted and applied to a variable length decoder (VLD) 308 and the
variable length codewords corresponding to motion vectors are applied to the variable length decoder (VLD) 306. The VLD 308 contains apparatus for performing variable length decoding, inverse run length decoding and inverse DPCM coding as appropriate
under the control of the controller 302. Decoded data from the VLD 308 are applied to an inverse DCT circuit 310 which includes circuitry to inverse quantize the respective DCT coefficients and to convert the coefficients to a matrix of pixel data. The
pixel data is then coupled to one input of an adder 312, the output of which is coupled to the video display ram 318 and buffer memories 314 and 316.

The VLD 306 includes circuitry to decode the variable length encoded motion vectors and to perform inverse DPCM coding of the motion vectors as appropriate under the control of the controller 302. Decoded motion vectors are applied to a motion
compensated predictor 304. Responsive to the motion vectors the predictor accesses corresponding blocks of pixels stored in one (forward) or both (forward and backward) of the buffer memories 314 and 316. The predictor provides a block of data (from
the one of the buffer memories) or an interpolated block of data (derived from respective blocks from both buffer memories) to a second input of the adder 312.

Decompression is performed as follows. If a field/frame of input video data is intraframe encoded there are no motion vectors and the decoded DCT coefficients correspond to blocks of pixel values. Thus for intraframe encoded data the predictor
304 applies a zero value to the adder 312 and the decoded DCT coefficients are passed unaltered by the adder 312, to the video display RAM where they are stored for readout according to normal raster scanning. The decoded pixel values are also stored in
one of the buffer memories 314 and 316 for use in forming predicted image values for decoding motion compensated frames (B or P).

If a field/frame of input data corresponds to a forward motion compensated P field/frame, the decoded coefficients correspond to residues or differences between the present field/frame and the lastmost occurring I frame. The predictor 304
responsive to the decoded motion vectors accesses the corresponding block of I frame data stored in either buffer memory 314 or 316 and provides this block of data to the adder wherein respective blocks of residues provided by the inverse DCT circuit 310
are added to the corresponding block of pixel data provided by the predictor 304. The sums generated by the adder 312 correspond to the pixel values for the respective blocks of the P field/frame, which pixel values are applied to the display RAM 318 to
update respective storage locations. In addition the pixel values provided by the adder 312 are stored in the one of buffer memories 314 and 316 not storing the I field/frame of pixel data utilized to generate the predicted pixel data.

For bidirectionally encoded (B) field/frames the operation is similar, except predicted values are accessed from the stored I and P pixel data stored in both buffer memories 314 and 316 depending upon whether the respective motion vectors are
forward or backward vectors or both. The generated B field/frame pixel values are applied to update the display RAM 318, but are not stored in either of the buffer memories, as B field/frame data is not utilized for generating other field/frames of
picture data.

FIG. 10A shows a combined priority deselect processor and decompressor. In FIG. 10A elements designated with the same numbers as elements in FIG. 10 are similar and perform like functions. The VLD's 308 and 306 have been substituted by DPCM's
409 and 408 which perform run length and DPCM decoding as appropriate. HP and LP data from the transport processor 25 are respectively applied to buffer memories 400 and 402. Transport headers H DATA from the HP and LP channels are applied to a
subcontroller 404 and communicated to the decompression controller 407. The subcontroller may comprise a subset of instructions of the controller 407. Responsive to the header data, the subcontroller aligns the data in the buffer memories 400 and 402
to correspond to the same data slices. The data in the buffer memories are applied to parallel to serial converters 401 and 403 which respectively provide serial data streams to respective input terminals of a multiplexer 405. Serial data from the
multiplexer 405 is applied to a VLD 406. VLD 406 accepts data a bit at a time and generates variable length decoded data to the decompression controller 407. Decoded video header data is stored in the controller 407, and decoded coefficients and motion
vectors are respectively applied by the controller to the DPCM's 409 and 408.

Controller 407 initially conditions the multiplexer 405 to pass HP data so that it accesses video header data first. The controller 407 reads decoded video header information, and responsive thereto establishes the appropriate decoding sequence. After the controller begins accessing data corresponding to macroblock coefficients, it counts the number of coefficient codewords decoded, and subsequent decoding j such codewords conditions the multiplexer 405 to pass data from the LP channel. Upon
receiving an EOB codeword in the LP data, the controller conditions the multiplexer to again pass data from the HP channel. The controller 407 is arranged to recognize missing LP data and effectively substitute EOB's therefor, that is, after counting j
HP decoded coefficient codewords, to proceed to the next HP block of data.

Referring to FIG. 10B, the control circuit 407, responsive to a channel change or loss of signal provided by the system controller performs an initialization procedure 700 wherein the VLD 406 is reset, the multiplexer 405 is conditioned to pass
HP data and the subcontroller is conditioned to test for GOP data. Thereafter a signal is sent 702 to load, e.g., a transport block of data in the buffer 400. A check 704 is made to determine if an HP transport block error occurred, and if so the next
transport block is loaded. In addition an internal error flag is set 703. If no error is set HP data is provided to the VLD and decoded 709 until either an EOB occurs or the jth codeword is decoded. If an error has been indicated for corresponding
blocks of data in the LP channel, an EOB code is inserted after each jth decoded codeword and the system continues to provide HP data to the VLD. In addition if the system has failed to align LP data with the HP data EOB codes are inserted after each
jth decoded HP codeword. Alternatively, if LP data has been aligned with HP data and no LP error flag has been set, after decoding the jth HP codeword a switch is made to process LP data (from 709 to 717). At this point the multiplexer is set to pass
LP data to the VLD. LP data is decoded 717 until an EOB occurs at which time decoding is switched back to the HP data.

Return to the check point 705. If an error flag has been set for the HP data, the system has to look for the next available data entry point which is identified by the RH data. Therefore the system checks 706 for an available RH. If none is
available the process is returned to 702 to get more data. If an RH is available, the TH is examined for its location 707, and the data in the buffer 400 is advanced to the byte boundary that immediately followed the RH.

The RH is examined 708 for the frame and slice identification and the associated CW#j. Thereafter decoding of Hp data commences, 709.

During initialization of the HP channel, the LP channel is similarly initialized. Thereafter a slice of LP data is loaded 710 in the buffer 402, and a check 712 is made for a transport block error. If an error is detected a LP error flag is set
and new data is loaded until no error is present. If no error flag has been set, a jump is made to 717. This jump indicates that the system is operating in the normal sequence, and decoding proceeds between HP and LP data as appropriate (709, 717).
Alternatively if an error flag has been set 713, then the system must search for the next valid data entry point. The current LP transport header is examined for a macroblock address or a RH 714. If none are available a jump is made to 710 and new data
is accessed. If one is available, the LP data is advanced to that entry point 715. Note if there is an HP error flag set the system ignores a MB entry point and advances to the entry point associated with a RH. The transport header is examined 716 for
the frame and slice identifiers. At this point the LP data is checked for coincidence with the HP data. If it lags the HP data a jump is made to 710 to advance the LP data. If there is no lag, a check is made for alignment of HP and LP MB's. If the
LP MB leads the current HP MB a wait is initiated. Conversely if the LP MB lags the HP MB the LP MB data is advanced to correspond with the current HP MB. The LP channel then waits for the jth HP codeword of the current block to be decoded.

As indicated previously, there may be record headers imbedded in the data stream that are not identified by the transport headers. These record headers are found by counting (709, 717) the number of macroblocks that are decoded. There are a
predetermined number of macroblocks in each slice, and therefore when this number of macroblocks are decoded the next occurring data will be recognized as e.g., two bytes of record header data. This data is read by the controller (708, 716) and used for
the next sequence of decoding operations. In addition there are a predetermined number of bytes in a transport block, and the bytes are counted (709, 717) to indicate when to access the next transport blocks (702, 710).

The FIG. 10B system is arranged to provide serial data to the VLD 406. However it should be recognized that a VLD may be selected which operates on variable length codewords occurring in parallel bit format. In this instance the parallel to
serial converters 401 and 402 may be eliminated. Preferably the data provide by the buffers will be tri-stated and the output ports of the buffers will be wire ORed to the input port of such VLD.

FIG. 11 illustrates exemplary modem circuitry for both the transmitting and receiving ends of the system. HP and LP data from the forward error correction circuits 15 and 16 are applied to respective 64 QAM modulators 400 and 401. The modulator
400 provides an HP analog signal with a -6 dB bandwidth of approximately 0.96 MHZ. This signal is applied to a 1.5 MHZ band pass filter 402 to eliminate high frequency harmonics, and then is applied to an analog signal summer 405. The modulator 401
provides an LP analog signal with a -6 dB bandwidth of approximately 3.84 MHZ. This signal is applied to a 6 MHZ band pass filter 404 to eliminate high frequency harmonics, and then is applied to an attenuator 406. The attenuator 406 reduces the
amplitude of the LP analog signal by approximately 9 dB relative to the HP analog signal. The attenuated LP signal is then coupled to the analog signal summer 405, wherein it is summed with the analog HP signal to produce a signal with a frequency
spectrum similar to the signal spectrum shown in FIG. 1. The combined signal is applied to a mixer 407 wherein it is multiplied by an RF carrier to frequency translate the combined signal to a frequency band that comports with a standard TV transmission
channel. The translated signal is then applied to a band pass filter 408, which tailors the spectral characteristics of the frequency translated signal to fit within the standard channel.

At the receiver, the transmitted signal is detected by a tuner/IF circuit 410 of conventional design, and applied to a PLL circuit 413 and an analog-to-digital converter (ADC) 412. The digitized signal is coupled to respective 64 QAM
demodulators 414 and 415. The demodulators 414 and 415 include band pass filters at their respective input connections to band limit the spectrum of the signals to be processed thereby to conform to the nominal signal spectrum of the HP and LP signals.
The demodulators 414 and 415 are of conventional QAM demodulator design and are responsive to clock signals provided by the PLL circuit 413. The PLL 413 develops the requisite clock signals via phase locking a signal developed by a voltage controlled
oscillator to one of the two carriers attendant the QAM signals.

The invention has been described in terms of an MPEG like signal however it should be appreciated that it is amenable to processing signals compressed in other formats and by other transforms. The only requirement on the type of compression is
that it provide data that can be prioritized in hierarchical levels, as for example sub band or pyramid transforms.